As wind turbines become increasingly widespread, there is a requirement for them to make significant contributions to the stability of the grid. An important aspect here is the provision of so-called balancing power. This is traditionally usually provided by conventional power stations, in particular coal- or gas-fired power stations, and to be precise in the form of power that can be additionally called up within a few seconds (primary balancing power). In order to be able to ensure such a rapid reaction, the corresponding power stations must be kept running permanently. This is expensive and entails a high consumption of fuel that is in many cases not used at all when balancing power is not called up. To reduce this cost, wind turbines are also included in the supply of primary balancing power.
A problem here is that for wind turbines the power output is determined by the wind and cannot be increased on demand, in contrast to conventional power stations. In order to be able to use wind turbines to provide primary balancing power, in spite of this limitation, it is known to obtain the required primary balancing power from the kinetic energy of the rotor. Various methods have been developed to do this:
In a first method, the operating point of the wind turbine is altered as a precautionary measure. The wind turbine is adjusted, by changing certain operating parameters and in particular the pitch angle of the rotor blades, in such a way that it is operated suboptimally (Janssens, N. et al: “Active Power Control Strategies of DFIG Wind Turbines”, IEEE Power Tech 2007, Lausanne, Switzerland, 1-5 Jul. 2007). It is thus possible to shift operating parameters on demand toward the optimum operating point and thus to output more power even with the same amount of wind and call it up as primary balancing power. A disadvantage of this approach is that, in normal operation (when there is no demand for balancing power), less power is generated by the wind turbine than is actually possible because of the suboptimal operating point.
In an alternative approach, the operating point is changed only when required, i.e. to draw off additional electrical power as primary balancing power. The speed regulator of the wind turbine is hereby modified in the short term and the target value for the power to be output is increased according to the primary balancing power which needs to be additionally output (for example by raising the target torque (Morren, J. et al: “Wind Turbines Emulating Inertia and Supporting Primary Frequency Control”, IEEE Transactions on Power Systems, vol. 21, no. 1, February 2006)). Although these and other known methods are simple, they do not take account of the influence of changes in the wind conditions during the provision of primary balancing power. These known methods are instead designed exclusively for stationary operating conditions and hence for unchanging wind conditions. This entails the disadvantage that in the event of non-stationary wind conditions—as are frequently encountered in practice—only a relatively poor provision of primary balancing power is achieved.